Not a matter of something you have done. It could simpley be the time of day you are testing the ph
The best times to test are midday and late afternoon as ph
will have had time to come up from the early morning.
Just before the lights go on, the ph
will be at it's lowest point. Without light, algaes produce CO2
much like houseplants and during the day produce O2
. The CO2
acts against the ph
and lowers it. Once the lights come on, the same algaes begin O2
production again and help increase the ph
I would also ask if you have a decent enough water flow at the surface for gas exchange and if the is no sump, are you using a glass cover on the tank?